1. Field of the Invention
The present invention relates generally to voltage-controlled oscillators operating mainly in a microwave or milliwave region and, more particularly, to a voltage-controlled oscillator capable of temperature compensation in a wide range of oscillation frequencies.
2. Background Art
A voltage-controlled oscillator whose oscillation frequency is controlled according to a voltage applied to the base of a transistor is being used. As such a voltage-controlled oscillator, one devised to obtain the same oscillation frequency at different temperatures has been proposed (see, for example, Japanese Patent Laid-Open No. 2005-102148).
FIG. 14 is a circuit diagram showing an example of a conventional voltage-controlled oscillator. An inductor 12 for series feedback is provided between the emitter of a bipolar transistor 11 and a grounding point. An output matching circuit 14 is provided between the collector of the bipolar transistor 11 and an output terminal 13. One end of a phase adjusting line 15 for satisfying a phase condition for starting oscillation is connected to the base of the bipolar transistor 11.
An inductor 16 and a variable-capacitance element 17 are connected in series between the other end of the phase adjusting line 15 and a grounding point. The inductor 16 and the variable-capacitance element 17 constitute an LC series resonance circuit. A frequency control bias circuit 21 applies to one end of the variable-capacitance element 17 a voltage for frequency control according to a control voltage input through a terminal 22.
In this voltage-controlled oscillator, the voltage applied to one end of the variable-capacitance element 17 is controlled with respect to temperature to control the oscillation frequency. In this way, variation in the oscillation frequency with temperature is corrected.
FIG. 15 is a diagram schematically showing oscillation frequency characteristics of the conventional voltage-controlled oscillator. For example, in a case where a voltage at a point A is applied to the two ends of the variable-capacitance element 17 at ordinary temperature, a voltage at a point B may be applied when a higher temperature is reached and a voltage at a point C may be applied when a lower temperature is reached, thus enabling the same oscillation frequency to be obtained at each temperature. In the voltage-controlled oscillator shown in FIG. 14, however, correction of the oscillation frequency with respect to temperature and frequency control at the same temperature are performed through the voltage applied to the same terminal and, therefore, complicated voltage control is required.
FIG. 16 is a circuit diagram showing another conventional voltage-controlled oscillator. A direct current blocking capacitor 18 is provided between a variable-capacitance element 17 and a grounding point. A temperature compensation bias circuit 23 applies to the other end of the variable-capacitance element 17 a voltage for temperature compensation according to a voltage generated by a temperature compensation bias generation circuit 24. In other respects, the configuration shown in FIG. 16 is the same as that of the voltage-controlled oscillator shown in FIG. 15.
The temperature compensation bias generation circuit 24 has a transistor 51, resistors 52 to 54 and terminals 55 and 56. A fixed voltage is applied from the terminal 55 to the collector of the transistor 51 through the resistor 52. A fixed voltage is applied from the terminal 56 to the base through the resistor 53. The emitter is grounded through the resistor 54. A voltage is output from the collector of the transistor 51 to the temperature compensation bias circuit 23.
FIG. 17 is a diagram schematically showing a characteristic of the voltage at a point X in FIG. 16 with respect to temperature. FIG. 18 is a diagram schematically showing a characteristic of the voltage between the two ends of the variable-capacitance element shown in FIG. 16. When the temperature rises, the collector current of the transistor 51 increases, the amount of voltage drop across the resistor 52 becomes larger and the voltage at point X becomes lower. When the voltage at the other end of the variable-capacitance element 17 is higher than the voltage at point X, the voltage applied between the two ends of the variable-capacitance element 17 increases and the oscillation frequency becomes higher. Accordingly, while the oscillation frequency of ordinary voltage-controlled oscillators becomes lower with increasing temperature, the voltage-controlled oscillator shown in FIG. 17 is capable of compensating for a reduction in oscillation frequency due to an increase in temperature. That is, the voltage applied to the variable-capacitance element can be automatically controlled so that a signal is output at the same frequency even when the temperature changes. Also, since the terminal 22 used for frequency control and the terminals 55 and 56 used for temperature compensation are separated from each other, there is no need for complicated voltage application with respect to temperature. In the voltage-controlled oscillator shown in FIG. 16, therefore, temperature control such as that in the voltage-controlled oscillator shown in FIG. 14 can be omitted or simplified.
For example, the signal frequency of on-vehicle radars is 76 to 77 GHz. Accordingly, the output frequency of voltage-controlled oscillators of on-vehicle radars is ordinarily ⅛ or more of 76 to 77 GHz. There is a tendency to increase the output frequency for the purpose of simplifying the system. In a voltage-controlled oscillator operating at such a high frequency, the influence of the assembly accuracy on the characteristics is large. It is, therefore, desirable from the view point of productivity that the voltage-controlled oscillator be provided in monolithic (MMIC) form. In the case of providing the voltage-controlled oscillator in MMIC form, a variable-capacitance diode (varactor diode) using the base-collector or base-emitter capacitance of a transistor is used as the variable-capacitance element.
The capacitance-voltage characteristic of a varactor diode, however, is determined from the viewpoint of matching to the high-frequency characteristics of the transistor provided as an oscillating element and, therefore, cannot be optimized with respect to the linearity of the oscillation frequency. FIG. 19 shows oscillation frequency characteristics of the voltage-controlled oscillator shown in FIG. 16 in such a case. A certain voltage offset is made independently of the control voltage. Therefore the control voltage with which temperature compensation can be performed is limited to only one point (point Y in FIG. 19) and variation in the oscillation frequency with temperature remains after compensation on the lower- or higher-voltage side thereof. That is, the range of oscillation frequency in which temperature compensation can be performed is considerably narrow.